Tobacco use, primarily through cigarette smoking, is responsible for about 5 million deaths annually, making it the largest cause of preventable mortality in the world, and nicotine is the component in tobacco that is responsible for the maintenance of smoking. This project builds upon the foundation laid in the first 5-year funding period of the Collaborative Genetic Study of Nicotine Dependence. We have identified several genes that affect susceptibility for nicotine dependence, such as the alphas nicotinic receptor, GABA4 genes. While identification of these genes represents significant progress toward our goal of understanding the molecular basis of nicotine dependence, many questions remain. The overarching goal of this project is to understand how these genes affect the susceptibility to develop nicotine dependence and related phenotypes, including alcohol and drug dependence, major depressive disorder and anxiety disorders. We are approaching this goal from several directions: expanded examination of nicotine dependence and related phenotypes, innovative genetic analyses, and incorporation of endophenotypic measurement of nicotine metabolism, which was performed on a subset of subjects. These goals will be accomplished through the integration and analysis of the extensive phenotypic, genetic, and endophenotypic data previously collected. Aim 1: To expand the genetic epidemiologic and association studies to examine different definitions of nicotine dependence, correlated phenotypes of alcohol dependence, major depressive disorder, anxiety disorder and other comorbidities. Correlated quantitative phenotypes will also be examined. Aim 2: To further investigate and model the complex genetic structure underlying risk for nicotine dependence. This work will include development and application of methods for genetic analysis of genegene and gene-environment interactions, using tools such as logistic regression, artificial neural networks (ANNs), and methods that account for the evolutionary history of the loci and haplotypes being analyzed. Aim 3: To incorporate and extend the endophenotypic measurement of nicotine metabolism into genetic analyses.